I do this similar warmup for every client I have, from beginners to division 1 college athletes. Obviously tweaks for specific needs.
Start with some movement for a warmup
Neck circles
Shoulder circles
Trunk twist
Light leg kicks
Ankle circles
For core exercises don't count the reps!!! Do it for time and feel the muscles working. Make them feel like they do something! That's when you know it's working. Connect with your body!
Prone plank :30-:90 sc
Supine bridge 1:00-2:00 mn
Crunches :30-90 sc
Back extensions :30-90 sc
Oblique crunches :30-90 sc
Repeat 1-3 times and good luck!!! Let me know how you did!!